1. Field of the Invention
The present invention relates to a pretreatment apparatus for raw materials for production of reduced iron.
2. Description of the Related Art
Generally, in producing reduced iron, the first step is to mix a powder of iron ore (iron oxide), a powder of coal (reducing agent), a powder of limestone (fluxing agent), and a binder such as bentonite, and compress and pelletize the mixture to form wet balls called green balls. Then, the wet balls are dried to some degree to form dry balls. The dry balls are heated to a high temperature in a reducing furnace, where iron oxide in the iron ore is reduced with the coal, as a reducing agent, to form reduced iron in pellet form.
Before the above-described compression and pelletization of various raw materials for production of reduced iron, pretreatment, for example, as shown in FIGS. 6 and 7 has been performed so far. FIG. 6 is a concept view of a conventional apparatus for pretreatment of raw materials for production of reduced iron. According to this apparatus, iron ore, coal, and flux, which are raw materials for production of reduced iron, are treated in the following manner: These materials are supplied in parallel from individual silos 51, 52, 53 to a single crusher 54 for mixing and crushing, and classified by a classifier 55. These steps are repeated to crush the raw materials into fine particles. The so finely crushed, mixed raw materials are further mixed in a mixer 57, stored in a feed bin 58, and fed from the feed bin 58 to a pelletizer 59 for production of raw pellets (pelletization). The raw pellets produced in the pelletizer 59 are dried in a dryer 61, and then carried and fed into a reducing furnace 63. FIG. 7 shows an equipment configuration ranging from the mixer 57 to the pelletizer 59 in FIG. 6. According to this configuration, powders of the raw materials mixed in the mixer 57 are stored and fed, for example, as follows: Total 16 of the feed bins 58 are provided for 8 of the pelletizers 59 at a rate of the two feed bins 58 perpelletizer 59 (one of the feed bins 58 for storage, and the other feed bin 58 for feed to the pelletizer 59). Storage and feed alternate such that the raw material powders are fed from two dedicated feed bins 58 to one pelletizer 59.
In performing mixing in the mixer 57, the hydrophilic iron ore and flux, and the hydrophobic coal, namely, the raw materials different in specific gravity and hardness, are mixed. As the mixer 57, a two-screw mixer has hitherto been used. This type of mixer is unsuitable for mixing the above-mentioned raw materials different in specific gravity and hardness, and it is difficult for such a mixer to feed homogeneously mixed raw materials stably to the pelletizer 59. To obtain a homogeneous mixture of the raw materials by use of the two-screw mixer, the screw shaft has to be fully long, or the rotational speed of the screws has to be fully increased. This results in a hike in the equipment cost or an increase in energy (power) consumption.
Furthermore, the iron ore is crushed, together with the coal and the flux, by means of the single crusher, regardless of the particle size of the iron ore. Thus, the type of mill is selected so as to be suitable for iron ore with high hardness. As a result, if the particle size of the iron ore is small, the crusher has overcapacity and is costly. Besides, the amount of the feed to the pelletization facilities is unstable, because it is easily affected by the particle size distribution of the raw materials before crushing. As shown in FIG. 7, therefore, the plural feed bins 58 have to be provided per pelletizer 59 in order to ensure an adequate amount of feed of the crushed raw materials to the pelletizer 59. This requirement further increases the equipment cost. equipment cost.
The present invention has been proposed in light of the foregoing problems with the conventional pretreatment of raw materials for production of reduced iron. It is an object of this invention to provide a pretreatment apparatus which can uniformize the crushed particle sizes of iron ore, coal, etc. different in hardness, stably supply homogeneously mixed raw materials, and simplify pelletization-side facilities.
A first aspect of the present invention, as a means of attaining the above object, is a pretreatment apparatus for raw materials for production of reduced iron, which applies pretreatment steps, including drying, crushing, classification, and mixing, to the raw materials for production of reduced iron, the raw materials comprising an iron material, such as iron ore, and other raw materials, such as coal and flux, and feeds the pretreated raw materials to a pelletization device via a feed bin, the pretreatment apparatus including a fluidized mixer for mixing the raw materials for production of reduced iron after the classification. According to this aspect, there can be realized a pretreatment apparatus which can stably supply homogeneously mixed raw materials comprising an iron material and other raw materials different in hardness, and simplify pelletization-side facilities.
A second aspect of the invention is the pretreatment apparatus for raw materials for production of reduced iron, wherein at least two of the fluidized mixers, one of the fluidized mixers for storage and the other fluidized mixer for supply, are provided for batch treatment. According to this aspect, constant-amount supply of the mixed raw materials is facilitated to ensure stably supply.
A third aspect of the invention is the pretreatment apparatus for raw materials for production of reduced iron, wherein when the iron material such as iron ore exceeds a predetermined particle size, the iron material such as iron ore is crushed by a dedicated crusher, and the other raw materials such as coal and flux are both crushed by a separate single crusher. According to this aspect, the crushing force of the crusher for crushing the iron material such as iron ore is utilized most effectively to decrease a loss of energy. Thus, the iron ore can be crushed to a uniform particle size much more efficiently than before.
A fourth aspect of the invention is the pretreatment apparatus for raw materials for production of reduced iron, wherein the crusher for crushing the iron material such as iron ore crushes an unrecovered iron material classified by a classifier, and returns the crushed iron material to the classifier. According to this aspect, only the iron ore that truly requires crushing can be crushed efficiently by the crusher.
A fifth aspect of the invention is the pretreatment apparatus for raw materials for production of reduced iron, wherein when the iron material such as iron ore has a predetermined or smaller particle size, the iron material such as iron ore is directly supplied from a raw material silo to a crusher for the other raw materials such as coal and flux, and premixed with the other raw materials, and the fluidized mixer is provided alone for continuous treatment of the premixed raw materials. According to this aspect, the equipment cost can be markedly decreased by simplification of the equipment.